The present technology relates to an image transmission system and an image transmission method. More particularly, the technology relates to an image transmission system and an image transmission method for preventing delays and stoppages of image transmission while reducing transmission load.
Heretofore, most surveillance camera systems have had surveillance cameras obtain image signals and had the images transmitted as analog signals to a camera control unit (called the CCU herein) via coaxial cables. This type of analog transmission setups has been extensively utilized. Inside the typical facility where the surveillance camera system is installed, the cables have been laid in every corner of the compound.
In recent years, there has been a need for image quality enhancement through digitization. However, the cost involved has turned out to be inordinately high when the densely laid cables inside the facility need to be replaced with new ones.
Given such high costs, techniques have been proposed to let digital signals be transmitted via the existing coaxial cables of the image transmission system.
Meanwhile, there are cases where the surveillance camera system includes a plurality of cameras that transmit their image signals over a single cable.
FIG. 1 shows a typical configuration of an image transmission system in which image signals from a plurality of cameras are transmitted via a coaxial cable.
In the image transmission system of FIG. 1, cameras 1-1 through 1-3 are interconnected with one another via a hub 2 using Ethernet cables. The hub 2 is connected to a modem 3. The cameras 1-1 through 1-3 acquire moving images and have them compression-coded in MPEG (Moving Picture Experts Group) format into image signals that are transmitted to a CCU 5 via the hub 2, modem 3, and coaxial cable 4. The CCU 5 transmits control signals that control the cameras 1-1 through 1-3 to these cameras via the coaxial cable 4, modem 3, and hub 2. As needed, the cameras 1-1 through 1-3 may transmit to the CCU 5 control signals corresponding to the control signals from the CCU 5, along with the image signals. The image signals transmitted to the CCU 5 are forwarded to a recording device and/or a reproduction device, not shown, for recording and/or reproduction (display).
The coded signals obtained through compression coding by the cameras 1-1 through 1-3 are each composed of a GOP (Group of Pictures) formed by an I-picture, a P-picture and a B-picture. The I-picture is a body of data made up of one frame of image information. The P-picture and B-picture are each a body of data formed by the difference in information between the preceding and the following frames. The data amount of the I-picture is significantly larger than that of the P-picture or B-picture. Thus there occurs periodically a peak of transmission load during image transmission every time the I-picture is transmitted from each of the cameras 1-1 through 1-3.
If the timings of I-picture transmission from the cameras 1-1 through 1-3 overlap with one another as shown on the left-hand side in FIG. 2, the transmission load on the coaxial cable 4 at each overlapping point in time rises prominently in pulsating fashion as indicated on the right-hand side in the same drawing. This can lead to the transmission load exceeding the maximum allowable transmission capacity of the coaxial cable 4.
The transmission load is typically prevented from overflowing the maximum allowable transmission capacity by having the transmitted signals being temporarily retained in a buffer attached to the modem 3. However, there remains a possibility that delays and stoppages of image transmission may still occur.
Meanwhile, there exists a technology whereby a single image is split into a plurality of image objects when coded and whereby the generation timings of the I-pictures of these image objects are adjusted to not overlap with one another temporally (e.g., see Japanese Patent Laid-open No. Hei 10-23427.